With weapons using a propellant that is substantially insensitive to temperature fluctuations, sighting systems need only to be adjustable to maintain the point of aim at varying distances from the target.
Propellants such as certain propellants used in the projectiles of extensible launchers present an additional problem in that they are sensitive to temperature changes with their efficiency decreasing as the temperature falls below a predetermined normal and increasing as the temperature rises above that level. The launchers, until extended, are water resistant and insulate the propellant against responding, but slowly, to temperatures to which the launchers are exposed.
Reference is made to the U.S. Pat. No. 3,279,072, to Choate et al, in which the problem of propellant efficiency in relation to temperature changes was solved by providing a rear sight which would respond to temperature changes with its thermally responsive element thermally insulated within the sight so as to reflect the temperature of the propellant thus automatically to maintain the proper point of aim.
The use of such sights required that adjustments for distance be made by means of a front sight. The resulting sighting system has proved to effectively eliminate problems in the changes in propellant efficiency due to changes in its temperature. A disadvantage is that, conventionally, compensation for distance is by means of a vertically adjustable rear "peep" sight. In addition, a sight in accordance with the patent necessarily had both the apertured sighting member and the thermally responsive member fully enclosed with the aperture moving vertically between front and rear windows and, as a consequence, such sights are relatively expensive.
Front sights in accordance with the copending application of Paul V. Choate, Ser. No. 617,377, now U.S. Pat. No. 4,607,445 meet the requirement of ensuring compensation for the efficiency of the projectile fuel as affected by the temperature thereof but impose a limitation on their use due to the fact that their height, when erected is such that corresponding changes in the rear sight are required and the resulting line of sight is objectionably high for most purposes.